1. Field of the Invention
The present invention relates to a focus detection apparatus and, more particularly, to a focus detection apparatus for performing focus detection by utilizing an object light beam passing through a photographing lens.
2. Description of the Related Art
Conventionally, various proposals have been made about a focus detection method of detecting an in-focus position by receiving a light beam from an object. First, as a method of detecting a contrast of an object, and then driving a photographing lens to maximize the contrast so as to obtain an in-focus point, "a hill-climbing servo system" described in, e.g., NHK Technical Research Vol. 17 No. 1 (65) pp. 21-37, and the like is known. As an invention associated with a camera comprising a focus detection apparatus adopting the hill-climbing servo system, one disclosed in Published Unexamined Japanese Patent Application No. 61-18271 is known.
This focus detection apparatus can perform a focus detection operation of not only a central portion of a photographing frame (to be referred to as a frame hereinafter), but also an object image at a peripheral portion of the frame, and similarly change a distance measurement area with reference to the central portion of a light-receiving surface. Therefore, according to this focus detection apparatus, a focus detection operation can be performed even if an object image is not located near the center of the frame, resulting in convenience for a user.
However, the camera disclosed in Unexamined Japanese Patent Application No. 61-18271 described above can only similarly change a distance measurement area with respect to the center of the frame. Furthermore, since the focus detection method complies with the "hill-climbing servo system" described above, a focal point is detected on the basis of MTF (Modulation Transfer Function) characteristics as contrast information for a focusing operation. In this case, detection precision may be impaired depending on a detection spatial frequency (to be simply referred to as a frequency hereinafter).
As a countermeasure against the above-mentioned problems, focus detection operations for a plurality of frequencies ranging from a low frequency to a high frequency may be performed. However, MTF characteristics at a peripheral portion of the frame are considerably changed depending on the frequency as compared to those at the central portion of the frame. For this reason, even if a focus detection at a high frequency is performed by the "hill-climbing servo system" on the basis of in-focus point data detected at a low frequency, it is difficult to detect a peak point of the MTF characteristics.
As for MTF characteristics for a standard object, deviations of peak points of MTF characteristics as in-focus points for respective frequencies at the central and peripheral portions of the frame are considerably different from each other at the central and peripheral portions of the frame in sagittal and meridional directions. Therefore, it is difficult to obtain a high-frequency in-focus point on the basis of a low-frequency in-focus point in a focusing operation for a peripheral portion, as described above.